Williams was originally a Labour MP - first elected to the House of Commons in 1964 - but left to found the Social Democratic Party (later the Liberal Democrats) in 1981. She served in government through the 1960s and 70s, but was defeated as an MP in 1983. She was awarded a life peerage in 1993 and retired in 2016. She was made a Companion of Honour in 2017.
She was the daughter of the author Vera Brittain and Sir George Catlin.